It has been known for some years that a part of the starch contained in the human diet can pass the small intestine without being digested. This fraction of food starch is called resistant starch. Different forms of starch have been found to be resistant to digestion. A classification of resistant starches has been given by Englyst and Cummings (Am. J. Clin. Nutr. (1987) 45 423-431). These authors distinguish between three types of resistant starches:
Type 1: Starch which is physically inaccessible to digestive enzymes (e.g. grains and seeds);
Type 2: Raw starch granules (present in e.g. raw potato and green banana); and
Type 3: Retrograded starch (found in cooked and cooled potatoes, bread and cornflakes for example).
Because it is not enzymatically digested in the small intestine, resistant starch reaches the colon where it becomes a substrate for fermentation by the gut's anaerobic microflora. This fermentation leads to increased nitrogen fixation, to the production of gases (e.g. H2, CH4 and CO2) and, importantly, to an increased formation of short chain fatty acids (SCFAs). The type and quantity of SCFA formed depends on the type of carbohydrate being fermented. However, the main SCFAs produced are acetate, propionate and n-butyrate.
Compared with other, less fermentable fibres such as non-starch polysaccharides, resistant starch produces a significantly greater molar quantity of n-butyrate (20-28% vs. 10-15%). This is considered important because of the major role that butyrate plays in the maintenance of a healthy digestive system. For example, butyrate is a prime substrate for energy metabolism in the colonocyte and acts as a growth factor to the healthy epithelium.
In normal cells, butyrate has been shown to induce proliferation at the crypt base, enhancing healthy tissue turnover and maintenance. In inflamed mucosa, butyrate stimulates regeneration of the diseased lining of the gut. In neoplastic cells, butyrate inhibits proliferation at the crypt surface, thus preventing the development of potential tumours. Moreover, models of experimental carcinogenesis in animals have shown butyrate's potential to modify a number of metabolic steps in the cell cycle such that early stages of cancer development may be counteracted while later stages may be slowed down. Thus, butyrate (and, by extension, resistant starch) has the potential to support the maintenance of a healthy digestive system and to reduce the risk of gut inflammation and colorectal cancer.
It is recognised that a healthy digestive system is essential to overall quality of life. The intestinal tract is indeed the organ through which nutrients that are required for growth, development and health are absorbed and through which undesired and waste substances are excreted. There has therefore been great interest by the food industry in the development of new high resistant starch products. To date, these include Actistar (Cerestar), CrystaLean (Opta Food Ingredients) and Novelose (National Starch and Chemical Company), for example.
It has now been found that products containing resistant starch could beneficially be used (instead of or in addition to other dietary fibres) in clinical nutrition. Due to the increased susceptibility of individuals in poor health and of those undergoing (or having undergone) treatment or surgery, it is important that any nutritional products administered to them be free from contamination. Food products are therefore preferably sterilised before administration. Sterilisation is usually achieved by UHT (ultra high temperature) treatment. Unfortunately, it has been found that resistant starch does not have very high thermal stability and, accordingly, that unacceptable levels are lost if submitted to high temperature sterilisation.
Methods of producing sterilised starch compositions exist in the art. However, none are suitable for the present use. In particular, none address the problem of resistant starch loss during high temperature treatment. U.S. Pat. No. 4,671,966, for example, discloses a method for the production of a thickener concentrate comprising sterilising a fatty material/starch product mix which is then emulsified with water under violent agitation and packaged. The application is not concerned with preventing starch degradation, let alone with trying to prevent the loss of resistant starch structures. Nor does the application relate to the preparation of sterile nutritional compositions. U.S. Pat. No. 4,671,966 is thus in a completely different field to that of the present invention.
FR 2686485 (Laiterie de Saint-Denis de l'Hotel) discloses a “sterilised” food composition, the ingredients of which are divided into two phases: a dry, powder phase (comprising the starch) and a sterilised, liquid phase. The two phases are packaged separately and mixed only upon use. The product can thus be stored for long periods without spoiling. However, there are several disadvantages to this method from the point of view of clinical nutrition. First, not all the ingredients are sterilised (only those comprised in the liquid phase). The risk of contamination (e.g. bacteria, moulds, etc.) is therefore not completely eliminated. In addition, the product is not ready to use. In clinical situations, it is often desirable to be able to administer nutrition directly to the patient, e.g. by enteral administration. The product described in FR 2686485 must be mixed before use and could therefore not be directly administered. This additional mixing step is not only time consuming, but it also introduces a risk of contamination which is not present when the product can be delivered straight to the patient from aseptic packaging.
FR 2636507 (Agro Investissement Developpement Aid) discloses a food composition, the ingredients of which are separated into two different categories: those which are heat-resistant and those, such as starch, which are heat-sensitive. The heat-resistant ingredients are sterilised by UHT treatment whereas the heat-sensitive ingredients are sterilised by ionisation. The two categories of ingredients are then combined in a mixing chamber and aseptically packaged. Although this method overcomes some of the problems of FR 2686485, it nonetheless has several disadvantages. First of all, it requires two types of machinery resulting in increased technical complexity and increased cost. The cost of ionisation, moreover, is relatively high, making the sterilisation procedure of FR 2636507 undesirably expensive. In addition, use of ionisation is tightly controlled and prior authorisation would therefore be required to carry out the process. This is usually difficult to obtain. Furthermore, because ionisation would have to be carried out separately, the procedure of FR 2636507 could not easily be used in a continuous production process and, therefore, on an industrial scale. The step of transferring the ionised starch powder from the ionisation device to the mixing chamber could also become a contamination risk unless carried out under strictly sterile conditions. Finally, the method of FR 2636507 relates to general starch compositions (e.g. flour) and does not consider the use of resistant starch compositions.
Consequently, there exists a clear need for an improved method of sterilising resistant starch and compositions containing resistant starch without causing unacceptable levels of denaturation. The present invention provides such a method.